ci: fall back to main version for PR site builds

The version guard rejects non-main/release branches, but build-site
also runs on PRs where the branch name is arbitrary. Default to "main"
for PR builds since deploy-aws is skipped anyway.

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/docs-orchestrator.yml

@@ -333,6 +333,8 @@ jobs:
             version="main"
           elif [[ "$branch" =~ ^release/ ]]; then
             version="v${branch#release/}"
+          elif [[ "${{ github.event_name }}" == "pull_request" ]]; then
+            version="main"
           else
             echo "::error::Unsupported branch for docs deploy: $branch (expected main or release/*)"
             exit 1